A current MIMO communication technology is gaining attention as a technique which efficiently uses a limited frequency band and achieves fast transmission. The MIMO communication technology uses a plurality of antennas for both a base station apparatus (hereinafter, referred to as a base station) and a terminal apparatus (hereinafter, referred to as a terminal). In this MIMO communication technology, a study is made on data transmission using preceding control in a terminal. In the precoding control, the base station estimates a channel condition between the base station and the terminal, from a receiving condition of a reference signal independently transmitted from each antenna of the terminal, selects a precoder which is optimal for the estimated channel condition and applies the precoder to data transmission.
Particularly, precoding control based on a transmission rank is applied to LTE-Advanced (Long Term Evolution-Advanced). Herein, a rank refers to the number of spatial multiplexing in space division multiplexing (SDM) and is the number of independent data transmitted at the same time. To be more specific, code books having different sizes are employed for respective ranks.
FIG. 1 is a diagram showing code books corresponding to respective ranks when there are four antennas. As shown in FIG. 1, rank 1 is associated with a code book having size 24, and the code book includes twenty-four candidate precoders. Rank 2 is associated with a code book having size 16 and the code book includes six-teen candidate precoders. Rank 3 is associated with a code book having size 12 and the code book includes twelve candidate precoders. Rank 4 is associated with a code hook having size 1 and the code hook includes one candidate precoder.
The base station receives a reference signal transmitted from the terminal, estimates a channel matrix from the received signal, and selects a precoder which is optimal for the estimated channel matrix from these fifty-three candidate precoders.
In a communication path such as mobile communication, having a relatively large channel variation, a hybrid automatic repeat request (HARQ) is applied for an error controlling technique. HARQ is a technique whereby the transmitting side retransmits data, and the receiving side combines the received data and the retransmitted data to improve error correction performance and achieve high quality transmission. As a HARQ method, adaptive HARQ and non-adaptive HARQ are under study. Adaptive HARQ is a method for allocating retransmitted data to any resource. On the other hand, non-adaptive HARQ is a method for allocating retransmitted data to predetermined resources. In an uplink of LTE, the non-adaptive HARQ scheme is employed among HARQ schemes.
A non-adaptive HARQ scheme will be described with reference to FIG. 2. In non-adaptive HARQ, the base station determines resources for allocating data in the first data allocation. The base station then reports transmission parameters to a terminal through a downlink control channel (PDCCH: Physical Downlink Control Channel). The transmission parameters include information such as allocated frequency resources indicating information on resource allocation, a transmission rank number, a precoder, and a modulation scheme/a coding rate. The terminal acquires the transmission parameters transmitted through the PDCCH and transmits first data, using a predetermined resource in accordance with the aforementioned resource allocation information.
The base station receives the first data and reports, to the terminal, a NACK corresponding to data which could not be demodulated in the first data, through a HARQ reporting channel (PHICH: Physical Hybrid-ARQ Indicator Channel). The terminal receives the NACK and controls retransmission by using the transmission parameters reported through the PDCCH, the parameters including information resource allocation and the like. Specifically, the terminal generates and transmits retransmission data, using an allocation frequency, a precoder, a modulation scheme, and the like, which are the same as those in the first transmission. The terminal changes an RV (Redundancy Version) parameter depending on the number of retransmission requests. The RV parameter represents a reading position in a memory (referred to as a circular buffer) for storing Turbo-coded data. For example, when the memory is equally divided into approximately four regions and tops of the areas are assigned zero, one, two, and three respectively, the terminal changes an RV parameter (a reading position) in order of zero, two, one, three, and zero depending on the number of retransmission requests.
Non-adaptive HARQ is often used together with Synchronous HARQ employing the constant transmission interval. In LTE, retransmission data is retransmitted eight subframes after the report of the NACK.
Non-adaptive HARQ is performed on a per predetermined control unit basis, the control unit is referred to as a codeword (CW). The CW is a control unit to which the same modulation scheme and coding rate are applied. As with the CW processed in a physical layer dealing with modulation or coding, the control unit may be referred to as a transport block (TB) since the control unit is processed in a MAC layer dealing with HARQ, and the CW may be distinguished from the TB. The present embodiment however employs uniform notation “CW” without a distinction therebetween hereafter.
In LTE, the transmission of one CW is generally applied to rank 1 (in transmission in a single rank) in the first transmission, and the transmission of two CWs is applied to ranks 2, 3, and 4 (in transmission in multiple ranks) in the first transmission. In the transmission in multiple ranks, CW0 is allocated to Layer 1 and CW1 is allocated to Layer 2 in rank 2. In rank 3, CW0 is allocated to Layer 1, and CW1 is allocated to Layer 2 and Layer 3. In rank 4, CW0 is allocated to Layer 1 and Layer 2, and CW1 is allocated to Layer 3 and Layer 4.
When retransmitting only CWs allocated to a plurality of layers, the terminal transmits one CW at a time in rank 2. To be More specific, when retransmitting CW1 in rank 3 and CW0 or CW1 in rank 4, the terminal transmits these CWs as one CW in rank 2.
Since the base station includes a larger number of antennas compared to the terminal, the base station is flexibly installed relatively. For this reason, a so-called multiuser MIMO, which assigns the same resource to a plurality of terminals, can be applied through an adequate process on a received signal in the base station. An example case will be described where the same resource is allocated to two terminals through the terminal having one transmitting antenna and the base station having two receiving antennas. This case can be equivalently treated as a MIMO channel with two transmitting antennas and two receiving antennas, and the base station can process a received signal. To be more specific, the base station performs a general MIMO received-signal process such as spatial filtering, canceller, and maximum likelihood estimation, thereby detecting respective signals transmitted from a plurality of terminals. With multiuser MIMO, the base station estimates interference values between terminals based on the channel condition between the base station and each terminal, and sets transmission parameters for the respective terminals by considering interference values, in order to more stably operate a communication system.
In non-adaptive HARQ, the base station sets transmission parameters in only resource allocation and the terminal transmits retransmission data, using a predetermined resource as described above. Accordingly, it is difficult for the base station to indicate an adequate precoder to the terminal at any timing. When retransmitting only a part of the first transmission data, the terminal may retransmit the data in a rank different from one reported in the first data allocation through PDCCH. Accordingly, such a case requires a rule for a rank in retransmission.
In a case of applying multiuser MIMO, the base station needs to estimate interference values between terminals upon resource allocation. Information of a precoder is required for estimating interference values between terminals. Using any precoder in the terminal in retransmission makes complicated the calculation of interference values between terminals in the base station.
Non-Patent Literature 1 discloses a technique to solve these problems. To be more specific, Non-Patent Literature 1 discloses a scheme which uses the same precoder for retransmission as that used in the previous transmission.
The scheme disclosed in Non-Patent Literature 1 will be described with reference to FIG. 3. FIG. 3 is a diagram showing a precoder used for the first transmission and retransmission in a case where a terminal has four transmitting antennas and transmits data in rank 3. FIG. 3 is an example where the base station selects a precoder of Index 0 (see FIG. 1) from the rank-3 codebook in first transmission. In this example, a case will be described where the base station could demodulate only one of a data sequence (CW0) transmitted in Layer 1 and data sequences (CW1) transmitted in Layer 2 and Layer 3 from the terminal.
(Case 1) A case where the base station could not demodulate CW0 but could demodulate CW1 (NACK, ACK). In this case, the base station transmits a NACK as a response signal to CW0 to instruct CW0 to be retransmitted, and transmits an ACK as a response signal to CW1 to instruct the transmission of CW1 to be stopped. These ACK and NACK are transmitted from the base station to the terminal through a PHICH.
In the scheme disclosed in Non-Patent Literature 1, the same precoder as one used in the previous transmission is applied to CW0 with response of the retransmission request (NACK). Accordingly, the terminal retransmits retransmission data corresponding to CW0, using only a part corresponding to Layer 1 in retransmission, the part being enclosed by a solid line, in a precoder of Index 0 in rank 3. The terminal changes an RV parameter between previous transmission and current transmission, and transmits the retransmission data.
(Case 2) A case where the base station could demodulate CW0 and could not demodulate CW1 (ACK, NACK). In this case, the base station transmits an ACK as a response signal to CW0 to instruct the transmission of CW0 to be stopped, and transmits an NACK as a response signal to CW1 to instruct CW1 to be retransmitted. These ACK and NACK are transmitted from the base station to the terminal through PHICH as with case 1.
In the scheme disclosed in Non-Patent Literature 1, the same precoder as one used in the previous transmission is applied to CW1 with response of the retransmission request (NACK). Accordingly, the terminal retransmits retransmission data corresponding to CW1, using only a part corresponding to Layer 2 and Layer 3 in retransmission, the part being enclosed by a dotted line, in a precoder of Index 0 in rank 3. The terminal changes an RV parameter between the previous transmission and the current transmission and transmits the retransmission data as with Case 1.